This invention relates to wireless transmission, and more particularly to circuitry in a wireless signal receiver for generating a receive criterion, i.e., for detecting the reception or non-reception of a signal, especially for receivers having alternating reception on several channels and facilities for seeking a free or an occupied channel.
There is often a need in receivers of wireless signals for binary detection of the reception or non-reception of a signal. It is generally customary to deduce the presence or absence of the signal from the intensity of the noise at the receiver output. In such cases, part of the output signal is led through a high-pass filter or a band-pass filter, and those frequencies contained in the noise which are above the speech band are processed into a noise criterion taking the form of a DC potential, the magnitude of which is approximately proportional to the intensity of the noise. By means of a comparison device which compares the noise criterion with an arbitrarily determined limit potential, a receive criterion is produced insofar as the noise criterion is lower than the limit voltage. Most often, the noise criterion is used solely for blocking the output of the receiver in the absence of a signal in order to avoid a disturbance due to the noise.
Since the noise intensity fluctuates, the noise potential must be integrated for a certain length of time before it is compared with the limit potential in order to avoid fluttering of the receive indication if the noise criterion is of the same order of magnitude as the limit potential. This integration is known to be achieved by means of a capacitor to which the noise potential is supplied across a resistor.
However, as a result of the insertion of such a capacitor, the beginning and end of the reception of a signal are detected with a lag dependent upon the strength of the arriving or disappearing signal. In the case of a newly arriving signal, the weaker it is, and thus the smaller the decrease in the potential of the noise criterion, the greater the lag. When the noise criterion is only slightly below the limit voltage, there is a very great lag. Conversely, when a strong signal disappears, hence when the decrease of the noise criterion is large, the lag is likewise great, whereas it is only brief upon disappearance of a weak signal, where the corresponding noise criterion is only slightly below the limit potential.
If the non-reception of a signal is supposed to be detected at a certain time, then in order to obtain a reliable result, it is necessary to wait for a time which exceeds the time it takes to detect the reception of a signal in the most unfavorable case, and which is therefore also long.
If the noise criterion is used only to control the blocking of the receiver output known as "squelch," it is above all the long lag time after the cessation of reception of a noise-free signal which is disturbing because the noise occurring at the receiver output during such a time lag is annoying. Contrary thereto, when reception of a weak and hence noisy signal is briefly interrupted, it is the quick response of the squelch which is disturbing because, on account of the delayed cancellation of the blocking in this case, the intelligibility suffers. Circuitry in which the dependence of the lag upon the strength of a disappearing signal is the opposite of the relationship , as described above, is disclosed in International (PCT) Application Publication No. WO 82/00552, for example.
In a receiver by means of which a group of channels is to be scanned for an occupied channel measuring periods are formed, each of these periods always begins with the switching over to a new channel. According to the prior art, in order to create a clear situation at the beginning of each measuring period, it is customary to apply briefly to the input of the comparator a (low) potential corresponding to the reception of a strong signal, whereupon this potential then, in accordance with the mentioned integration time constant, tends to the value corresponding to the noise occurring. As soon as this rising potential exceeds the limit voltage, a pulse is transmitted, brought about by the comparator, for switching the receiver to another channel and thus for initiating a new measuring period.
If, in this case, a weak signal is received, which is not supposed to be acknowledged and where the potential corresponding to the noise is only slightly above the limit voltage, it takes a long time from the beginning of the measuring period to the transmission of the mentioned pulse, which is a disadvantage when a fairly large number of channels are supposed to be scanned.
In an arrangement for seeking free channels, problems corresponding to the aforementioned time problems likewise arise, according to the prior art, when signals are received where the potential derived from the noise voltage is only slightly below the limit voltage.
An arrangement operating according to these principles and hence suffering from the mentioned disadvantages in the speed of the mode of operation, intended exclusively for scanning for occupied channels, is described in European Patent Application Publication No. 0 027 365.
It is an object of this invention to provide improved circuitry by means of which, during alternating reception on different channels, the time for detecting the reception of a signal or for detecting the non-reception of a signal is substantially reduced as compared with conventional and obvious circuit arrangements, although in both cases the reception of weak signals is also taken into consideration.